This invention relates to an optical read/write apparatus, more particularly to an optical read/write apparatus with improved tracking performance.
In an optical read/write apparatus, a light beam from a light source such as a laser diode is focused by an objective lens onto a spot on an information track on a recording medium such as an optical disk. In write mode, information is recorded on the track by, for example, modulating the intensity of the beam. In read mode, information already recorded on the track is reproduced by converting the light reflected from the spot to an electrical signal. Different beams can be used for reading and writing, or the same beam can be switched between read and write modes. In both read and write modes the objective lens is moved according to tracking error and focusing error signals derived from the reflected light, to keep the spot precisely in focus and centered on the information track.
Most prior-art optical read/write apparatus uses the push-pull method of tracking control, in which light reflected from the recording medium is received by a split photodetector, the difference between the signals from the two halves of the photodetector being used as the tracking error signal. A disadvantage of this method is that it is prone to offset effects when the recording medium is tilted, or when the objective lens is moved to follow an information track. Under these conditions, a zero tracking error signal occurs not when the spot is centered on the information track, but when it is offset by a certain distance from the track center. A tilt of as little as 1.degree., or a tracking motion of as little as 100 .mu.m by the objective lens, can cause an unacceptably high track offset.
An alternative method of tracking control is the radial wobbling method, in which a slight oscillatory motion is imparted to the spot, causing it to wobble in a direction perpendicular to the information tracks, and tracking is controlled by detecting the response of the reflected light to the wobble. It is known that the radial wobbling method almost completely solves the problem of track offset. Unfortunately, this method cannot be used to control tracking during both reading and writing, due to the impracticality of synchronizing the reading wobble with the writing wobble. In particular, the radial wobbling method is unsuitable for apparatus that uses a single beam for both reading and writing.